My Bigfoot is a SB on a LB. I made a spacer out of 2X8. One right at the front of box, the second in the pocket that GM puts in the side of the box,then two pieces to line up with the bump stops. Then I had to add a piece of 1X6 on so the camper didn't hit tail lights.
The space is handy. I carry my generator in there with a spare gas can. Had 2 5 gallon diesel cans and my 5 lb> propane tank as well.
It also allows me to stuff things beside the camper in front of the wheel wells. Just have to be sure that whatever goes in can be reached to get it back out.
Brian